One conventional type of interferometer, known as a "Mach-Zehnder interferometer," includes an input beamsplitter which divides the amplitude of an incoming wavefront into a reference beam and a measurement beam. The reference beam propagates through a reference arm of the apparatus, the measurement beam propagates through a measurement arm of the apparatus, and the two beams are recombined at an output beamsplitter. Fringes of the recombined beam are observed to reveal optical path differences between the reference and measurement arms (for example, at times when a gas flow, flame, or other object of interest is positioned in the path of the measurement beam along the measurement arm).
When the object of interest is the input laser beam itself, the reference arm can include a means for spatially filtering the reference portion of the input beam to produce a high quality spherical wave. The spherical wave is then collimated, and finally recombined with the "measurement" portion of the input beam. Since the reference wavefront is produced from the beam under test, this conventional type of Mach-Zehnder interferometer is known as a "self-referencing Mach-Zehnder interferometer" (or "SRMZ").
It would be desirable to design an SRMZ to be capable of extremely accurate wavefront measurements (for example, measurements with accuracy of at least L/10 peak-to-valley, where L is the input beam wavelength) over a broad range of input beam wavelengths (for example, over at least a 300 nm wavelength range), without the need to readjust any optical components. It would also be desirable to design such an SRMZ to be polarization insensitive, and to be capable of accurate wavefront measurements on radiation from broadband sources as well as closely spaced laser line sources. However, until the present invention, it was not known how to design an SRMZ having the features mentioned in this paragraph.